LTC1629 LINER [Linear Technology], LTC1629 Datasheet - Page 13

• Manufacturer Part Number: LTC1629
• Description: PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
So the number of phases used can be selected to minimize
the output ripple current and therefore the output ripple
voltage at the given input and output voltages. In applica-
tions having a highly varying input voltage, additional
phases will produce the best results.
Accepting larger values of I
inductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is I
= 0.4(I
is the total load current. Remember, the maximum I
occurs at the maximum input voltage. The individual
inductor ripple currents are constant determined by the
inductor, input and output voltages.
Inductor Core Selection
Once the values for L1 and L2 are known, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of more expensive
ferrite, molypermalloy, or Kool M
loss is independent of core size for a fixed inductor value,
but it is very dependent on inductance selected. As induc-
tance increases, core losses go down. Unfortunately,
increased inductance requires more turns of wire and
therefore copper losses will increase.
Kool M is a registered trademark of Magnetics, Inc.
V
V
OUT
IN
OUT
Figure 3. Normalized Peak Output Current vs
Duty Factor [I
)/N, where N is the number of channels and I
N
k
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
where k = 1, 2, …, N – 1
0.2
U
RMS
0.3
DUTY FACTOR (V
0.3 ( I
U
0.4
0.5
L
O(P–P)
allows the use of low
OUT
0.6
W
®
/V
)]
IN
0.7
cores. Actual core
1-PHASE
2-PHASE
3-PHASE
4-PHASE
6-PHASE
)
0.8
1629 F03
0.9
U
OUT
L
L
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive than
ferrite. A reasonable compromise from the same manu-
facturer is Kool M . Toroids are very space efficient,
especially when you can use several layers of wire. Be-
cause they lack a bobbin, mounting is more difficult.
However, designs for surface mount are available which
do not increase the height significantly.
Power MOSFET, D1 and D2 Selection
Two external power MOSFETs must be selected for each
controller with the LTC1629: One N-channel MOSFET for
the top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
The peak-to-peak drive levels are set by the INTV
age. This voltage is typically 5V during start-up (see
EXTV
old MOSFETs must be used in most applications. The only
exception is if low input voltage is expected (V
then, sublogic-level threshold MOSFETs (V
should be used. Pay close attention to the BV
cation for the MOSFETs as well; most of the logic-level
MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
input voltage, and maximum output current. When the
LTC1629 is operating in continuous mode the duty factors
for the top and bottom MOSFETs of each output stage are
given by:
Main Switch Duty Cycle
Synchronous Switch Duty Cycle
CC
Pin Connection). Consequently, logic-level thresh-
LTC1629/LTC1629-PG
DS(ON)
, reverse transfer capacitance C
V
V
OUT
IN
V
IN
GS(TH)
–
V
DSS
IN
V
OUT
IN
13
CC
specifi-
< 5V);
< 3V)
volt-
RSS
,